Page 1 Effects of Different Lifting Cadences on Ground Reaction Forces during the Squat Exercise Jason R. Bentley 1 , William E. Amonette 2 , John K. De Witt 1 , and R. Donald Hagan 3 1 Wyle Life Sciences Group, Houston, TX 2 University of Houston – Clear Lake, Houston, TX 3 NASA – Johnson Space Center, Houston, TX Corresponding Author: Jason R. Bentley, M.S., CSCS Wyle Life Sciences Group 1290 Hercules Dr. Ste. 120 Mail Code: WYLE/HAC/261 Houston, TX 77058 Voice: (281) 483-7139 Fax: (281) 483-4181 E-mail: [email protected]Running Title: Cadence Variation and Squatting Forces https://ntrs.nasa.gov/search.jsp?R=20080013257 2020-03-21T06:09:30+00:00Z
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Effects of Different Lifting Cadences on Ground Reaction Forces during the Squat Exercise
Jason R. Bentley1, William E. Amonette2, John K. De Witt1,
where GRF is the ground reaction force, mbody is the mass of the body, and abody is the
acceleration of the body.
The inertial forces associated with the system (FIsystem) are calculated as:
FIsystem = GRF + msystemg = msystemasystem (5)
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where msystem is the bar mass (mbar) plus body mass (mbody), and asystem is the acceleration of the
system (abar and abody). The derivations for these equations are outlined in the Appendix.
The start of each repetition was defined as the first of one hundred consecutive
decreasing bar position samples (0.5 sec of data) during which the bar velocity was negative and
the bar moved at least 3 cm. The midpoint of each repetition, or the point where the bar motion
changed from downward to upward, was defined as the minimum position value of the barbell.
The end of each repetition was defined as the last of fifty consecutive increasing position
samples during which the bar velocity was positive and the bar moved at least 3 cm.
All inertial force values were normalized to body mass to account for subject differences.
The maximum, minimum and range of inertial forces for the body, bar and system were found
for each phase of each repetition.
Statistical Analysis
All statistical procedures were completed using Statistica software (StatSoft, Inc., Tulsa,
OK). A set of squats consisted of three repetitions, except for a few cases when the data
acquisition system produced errant data, which was identified as a statistical outlier (>2 SD). In
these cases, the corresponding sets consisted of only two repetitions. Each subject performed a
total of 9 repetitions of squats, and the mean of the 9 repetitions performed at each cadence (FC,
MC, and SC) was calculated for each variable of interest. Measures of squat kinematics, GRFR,
and system inertial force were analyzed using a two-way ANOVA with repeated measures in the
cadence. Tukey’s post-hoc comparisons were performed to discern significant differences
between cadences. The criterion for statistical significance was set at p<0.05.
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RESULTS
No differences were found between cadences for bar displacement, peak knee angle, or
peak hip angle (Table 2), indicating that subjects achieved the same depth during the squat
regardless of the timing condition. As expected, differences were seen in ascent times between
the SC and both FC (p=0.0002) and MC (p=0.0004) (Table 2). No statistical differences were
noted in ascent time for FC and MC (p=0.07), although the mean ascent time was 0.22 s faster
during the FC.
[Insert Table 2 here.]
Figure 1 shows a typical GRFR during a slow, medium, and fast cadence squat, while
Figure 2 illustrates the peak and range of GRFR and system inertial forces. Peak GRFR was
greater during FC squats than MC (p=0.0002) and SC (p=0.0002). No differences in peak GRFR
were found between MC and SC squats.
The range of GRFR differed significantly between each cadence. The FC squats had the
highest range of GRFR and the SC squats had the lowest, with the majority of the difference
primarily in the nadir rather than the peak. Similar trends were seen in peak system inertial
force, where FC was significantly greater than MC (p=0.0002) and SC (p=0.0002). No statistical
differences were found between MC and SC (p=0.1112). The range of system inertial force
measures was significantly greater for FC squats compared to either MC or SC squats, and for
MC squats compared to SC squats.
[Insert Figure 1 here.]
[Insert Figure 2 here.]
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Figures 3 and 4 show the peak and range, respectively, of inertial forces generated by the
body and barbell for each cadence, given that the mass of the external load provided by a barbell
and the mass of the body were approximately equal. Irrespective of cadence, the peak and range
of inertial forces generated by the body were significantly greater than those generated by the
bar.
[Insert Figure 3 here.]
[Insert Figure 4 here.]
DISCUSSION
This study quantified the impact of lifting cadence on the inertial forces associated with
the parallel squat. In addition, the inertial forces of the body and barbell were compared to
determine if they were equal given identical static loads. Subjects performed squats using a
barbell loaded nearly equal to their body weight at fast, medium and slow cadences. The results
of this study indicate that squat cadence significantly affects the GRFR and the associated inertial
forces. The squats performed at faster cadences resulted in greater peak and range of GRFR than
those at slower speeds; furthermore, descent time significantly affects the forces developed,
regardless of ascent time. This is reinforced by the fact that differences in the range of GRFR
were primarily in the nadir, thus the differences were primarily from the descent, not the ascent.
The differences in GRFR are due to the inertia of the system.
The peak and range of GRFR and system inertial forces increased as movement time
decreased. This result was expected, since at any given time the GRFR is the sum of the GRF
due to gravity and the GRF due to the acceleration of the system being supported. Because
gravity remains constant, any variation in GRFR will be due to the system’s motion, and are
Deleted: ¶
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reflected as the system inertial forces. Since the inertial force is due to acceleration, and
decreases in movement time with identical displacement requires an increase in acceleration,
faster squats should generate higher inertial forces.
Although the FC and MC ascent times were somewhat dissimilar (FC=1.03 s, MC=1.25
s), the descent times were very different (FC=1.21 s, MC=3.23 s). This suggests that the time of
descent primarily affects the forces experienced by the body during ascent, which indicates that
the faster descent time accentuated the stretch reflex. However, with similar ascent times, it
should be expected that the peak inertial forces developed would be similar between these two
cadences. However, this was not the case. The FC squats resulted in greater inertial forces than
the MC. This suggests that the peak force developed during the ascent is influenced by the rate
of descent, highlighting the importance of the rate of descent on the stretch reflex response.
Since the peak force occurs near the initiation of ascent (Figure 1), it is possible that this is due to
the greater acceleration occurring when the downward velocity of the system was changed
quickly to upward velocity.
While the FC had significantly greater peak GRFR than the MC and SC, examination of
Figure 1 suggests that the range of GRFR experienced during the FC may be affected by the
decreases in GRFR at the start and end of the repetition. The decrease at the start of the repetition
probably occurs because the body becomes temporarily unloaded during the sudden lowering of
the barbell. The decrease at the end of the motion may occur because the inertia of the body and
barbell cause an unloading effect when all of the joints return to neutral positions. While it is
intuitive that lifting greater peak loads will result in greater strength gains, it is unclear if this
increase in range of load is of any physiological benefit. Also, for experienced weightlifters,
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lifting at faster cadences can be done safely, but faster cadences could pose safety risks if the
subject does not possess ability to develop the rate of muscular force necessary to control rapidly
changing forces.
We found that the peak forces realized by the body are highly affected by movement time
due to inertial effects of the body and barbell motion. It has been shown that squat execution
speed affects rate of muscle force development (16). Training with faster concentric squats
results in greater improvement in power than slower squats (7,11). It is possible, due to the large
ranges and peak force magnitudes associated with high velocity movements, that the benefits of
both high load magnitude training and high velocity training are available when lifting at faster
cadences. This benefit might be accentuated by increasing the velocity of both the eccentric and
concentric phase of the squat.
Since this study required the subjects to reverse their motion immediately from the
descent to the ascent, it is not clear if this finding would occur if the subject had paused during
the reversal. The increased rate of descent resulted in an enhanced force development of the
musculature used during the ascent. It is known that rapid skeletal muscle fiber lengthening
results in activation of the stretch-shortening cycle, producing an involuntary contraction known
as the stretch reflex (1,6,9). This neuromuscular characteristic is often conditioned with
plyometric exercise (3,15). It is also known that force and power can be affected by slower or
faster lifting cadences (7,8,13), and that training at slower or faster cadences can affect strength
and power gains (11,12). Perhaps future study should focus on the descent cadence and
determine if conditioning with a fast descent and fast ascent results in greater human muscle
power development than conditioning with a slow descent and fast ascent.
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Although the barbell bar may flex slightly during a squat, it is relatively stiff and was
considered to be a rigid body. The inertial forces subjected to the barbell are predictable using
Newton’s second law, and were found by modeling the barbell as a single point in space.
However, the human body is a much more complex system consisting of multiple segments of
various masses moving at varying rates. Since inertial force is related to the mass of the object,
it seems reasonable to assume that the contributions of the body and barbell to the overall system
inertia should be similar given that the barbell mass was approximately equal to the body mass.
It is interesting that the inertia of the body was not equal to the inertia of the barbell . In fact, the
body inertia was larger than the bar inertia regardless of squat cadence. Since the mass of the
body and barbell were approximately the same, it is possible that the unequal inertial forces were
due to the interactions and various rotations of the segmental masses of the body, which cannot
be simplified as the motion of a single point mass.
During the squat, the lower leg, thighs and trunk all undergo significant translations and
rotations (5). Each of these segments has a mass, and associated inertial forces. It was not
possible, based on the methodology of this experiment, to determine the inertial affects of the
various body segments. However, it can be speculated that the squat motion induces inertial
forces on each body segment that may affect the respective body segment differently, and that
the sum of these forces does not accurately represent the inertial force computed using a single
point mass to represent the body.
In conclusion, our findings demonstrate that the peak GRFR and the inertial forces during
a squat are greater when performed at faster cadences. Furthermore, the force during the ascent
is affected by the descent cadence. Since these results suggest that inertial force produced
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during a squat exercise is affected more by the movement of the body than the barbell, future
studies should attempt to determine how the relative inertial contribution of the barbell and body
to the overall system forces is affected by the magnitude of the external load.
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PRACTICAL APPLICATION Rehabilitation and training prescriptions should account for the descent cadence of the lift
performed. While this study cannot determine the long-term physiological effects of performing
squat exercises at different cadences, these results suggest that athletes, clients, or patients who
perform squats at greater movement velocities will be exposed to greater magnitudes and rates of
musculoskeletal loading. This may provide the experienced weightlifter the same benefit of
training with increased resistance, namely muscle and bone formation, while operating with a
decreased risk because there is less external load to control. While the barbell loads may be
lower during faster velocity training, it is possible that the inertial effects of the motion result in
peak forces similar in magnitude to training at slower cadences with a higher load.
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REFERENCES
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APPENDIX: Method used to determine inertial forces
The inertial force (FIi) acting upon the object i is expressed as:
FIi iii m aF == ∑ (6)
The external forces acting upon the object include the gravitational force (mig), forces
due to muscular activity, and reaction forces from other objects.
The inertial force of the barbell, FIbar, is found using the equation:
FIbar = Fbar + mbarg = mbarabar (7)
where Fbar is the net force exerted on the bar by the person, mbar is the mass of the bar, g is the
acceleration of the bar due to gravity (constant -9.807 m/s2), and abar is the acceleration of the
bar in space (See Figure 5).
[Insert Figure 5 here.]
The forces acting upon the body’s center of mass (CM, Fbody) can be summed using
equation 6 to determine the inertial force acting upon the body, FIbody.
FIbody = mbodyabody (8)
Figure 6 shows the FBD of all the forces acting upon the body. If the body is considered
a single lumped mass, at any given time the forces acting upon the entire system include the Fbar
and the GRF. The negative Fbar term reflects the reaction force between the body and barbell,
and is equal and opposite to the force of the body acting upon the bar expressed in Figure 5.